Differentiation requires the regulated synthesis of several proteins which allow cells to function in the specialized state. Among these are enzyme proteins which are usually not abundant but perform important functions in differentiated cells. The mechaisms which regulate synthesis of these enzymes during differentiation are poorly understood because it is difficult to isolate genes encoding minor species. UDPG pyrophosphorylase, the most extensively studied enzyme of Dictyostelium, is the only one which is known to be essential for normal development. We have isolated, by molecular cloning, sequences which encode this protein and propose to use the cloned sequence to determine the structure of the UDPG pyrophosphorylase gene and its pattern of transcription. The origin of the observed heterogeneity in UDPG pyrophosphorylase will be examined at the nucleic acid level. The nature of the mutations which affect enzyme expression will be studied in detail using the cloned sequences.